feh(1): Mention --action execution in --(un)loadable documentation

Previously, the fact that --action interacts with --(un)loadable was only
mentioned in the documentation for --action, thus posing an elevated risk for
misunderstandings.

Closes #741

man/feh.pre

@@ -575,6 +575,9 @@ Useful in scripts to hunt out images of a certain size/resolution/type etc.
 Don't display images.
 Just print out their names if imlib2 can successfully load them.
 Returns false if at least one image failed to load.
+When combined with
+.Cm --action ,
+the specified action will be run for each loadable image.
 .
 .It Cm --max-dimension Ar width No x Ar height
 .
@@ -923,6 +926,9 @@ See
 Don't display images.
 Just print out their names if imlib2 can NOT successfully load them.
 Returns false if at least one image was loadable.
+When combined with
+.Cm --action ,
+the specified action will be run for each unloadable file.
 .
 .It Cm -V , --verbose
 .